#3 The SummerSlam card is stacked in itself
On paper, this year's SummerSlam card itself is packed and stacked at the same time - and this is despite missing many stars on the card and matches that were rumoured being cancelled. Just to put it in perspective, here are some of the marquee matches for SummerSlam this year:
Seth Rollins vs Brock Lesnar - Universal Championship
Kofi Kingston vs Randy Orton - WWE Championship
AJ Styles vs Ricochet - United States Championship
Becky Lynch vs Natalya - RAW Women's Championship
Charlotte Flair vs Trish Stratus
Dolph Ziggler vs Goldberg
These are just a few of the ten matches added to the card and it's obvious that there are likely going to be a couple of more matches added to the pre-show - probably including the Intercontinental and Tag Team titles as well.
Given that there are this many matches, it probably didn't make sense for WWE to stuff the card more than it needs to be.